In the mass spectrometry as described above, a method in which a compound constituting a sample to be measured is analyzed by ionization is called an ionization method.
Various ionization methods of a compound have been proposed, such as an Electron Ionization (EI) method, a Chemical Ionization (CI) method, a Fast Atom Bombardment (FAB) method, an Inductively Coupled Plasma (ICP) method, a Laser Desorption (LD) method, Thermospray method, an Electrospray Ionization (ESI) method, and an Atmospheric Pressure Chemical Ionization (APCI) method.
In mass spectrometry, a hybrid type analytical method has been widely performed in which a detection means is used in combination with another isolation means for a substance to be measured. In all the above techniques, it is naturally understood that effective ionization of a sample to be measured is very significant from a technical point of view to improve the accuracy and sensitivity of the analysis. In particular, in the hybrid type analytical method described above, a mass spectrometer is often provided in combination with an isolation means for a sample to be measured, such as gas chromatography (GC), liquid chromatography (LC), and capillary electrophoresis (CE).
In this hybrid type analytical method, it is important that an ionization means be provided which can efficiently ionize individual chemical components while decomposition of a compound to be measured, which is contained in a sample to be measured and which is isolated by the above isolation means, is suppressed as small as possible.
In the case in which a mass spectrometer is used in combination with an isolation apparatus other than gas chromatograph (GC), such as liquid chromatograph (LC) or capillary electrophoresis (CE), the electrospray ionization (ESI) method and the atmospheric pressure chemical ionization (APCI) method may be mentioned as efficient ionization methods. In the electrospray ionization method, when a compound to be measured, which is to be ionized, is a polar substance, the compound has a relatively low ionization potential or a high proton affinity or electron affinity, and thus the ionization is easily performed; however, when the compound is a non-polar substance, the ionization is not easily performed in many cases. In the atmospheric pressure chemical ionization method, an evaporated solvent is ionized by corona discharge and enables a substance to be easily ionized, and hence some non-polar substance can also be ionized; however, the above substance is required to have a higher proton affinity or electron affinity than that of the solvent or a lower ionization energy (also called an ionization potential) or acidity than that of the solvent. In order to improve the above problem in that non-polar substances are not easily ionized, an ionization method has been proposed in which an element with high ionization energy such as helium or argon is used together with a non-polar substance.
As a technique for improving mass spectrometric properties by the method described above, for example, a technique disclosed in the following patent document 1 may be mentioned.
The patent document 1 describes that, for example, non-polar molecules such as dioxins and PCB, which are minor constituents, are not easily ionized by an electrospray ionization method which is a conventional technique, and thus these molecules have been hardly detected.
Then, in the patent document 1, basically as a means for increasing efficiency of the ionization, the use of high frequency plasma generated by a microwave resonator is proposed. In addition, it is also proposed that a port for supplying a helium or argon gas is provided for a spray device for spraying liquids used in this technique, together with a sheath liquid supply port provided around a port for supplying an isolated sample to be measured for facilitating evaporation of the sample to be measured, so as to increase the ionization efficiency of components of the sample to be measured by the supply of the gas (see [0009] of the patent document 1).
Subsequently, it is described that by supplying the gas, plasma thereof is generated, and ionization of a non-polar compound can be improved which cannot be ionized by a conventional method due to its high ionization potential (see [0015] of the patent document 1). According to this document, since a helium gas and an argon gas are not used in an excited state but are in the form of plasma, it is estimated that the ionization is not caused by Penning effect.
Incidentally, Penning ionization is a phenomenon in which a metastable excited atom takes an electron out of a second atom with an ionization potential, whose energy is lower than that of the metastable atom. The electron is placed into a vacant ground state of the metastable atom, and the second atom is ionized.
In addition, a method has been reported in which an excited gas generated beforehand is allowed to act on a sprayed sample so as to increase ionization efficiency of the sample. The method which uses the principle of the Penning effect facilitates ionization of a sample to be measured which is isolated by another isolation means, and the sample thus ionized is supplied to a mass spectrometer. For example, a method developed by Zhu may be mentioned (disclosed in the patent document 2) in which a long cylindrical discharge chamber provided with a coil is used. Further to that, according to a method developed by Bertrand et al. (disclosed in the patent document 3), an interface to be connected to a mass spectrometer is provided. The interface, for example, is two chambers which include a gas mixing chamber and a discharge chamber. Hence, it is difficult to use with an apparatus of an atmospheric pressure chemical ionization (APCI) method using corona discharge electrodes, an electrospray ionization (ESI) method, or the like, which is one of the most widely used ionization methods of mass spectrometry.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2001-108656
[Patent Document 2] U.S. Pat. No. 5,192,865
[Patent Document 3] U.S. Pat. No. 6,124,675